> This change adds a fast-locking scheme as an alternative to the current > stack-locking implementation. It retains the advantages of stack-locking > (namely fast locking in uncontended code-paths), while avoiding the overload > of the mark word. That overloading causes massive problems with Lilliput, > because it means we have to check and deal with this situation when trying to > access the mark-word. And because of the very racy nature, this turns out to > be very complex and would involve a variant of the inflation protocol to > ensure that the object header is stable. (The current implementation of > setting/fetching the i-hash provides a glimpse into the complexity). > > What the original stack-locking does is basically to push a stack-lock onto > the stack which consists only of the displaced header, and CAS a pointer to > this stack location into the object header (the lowest two header bits being > 00 indicate 'stack-locked'). The pointer into the stack can then be used to > identify which thread currently owns the lock. > > This change basically reverses stack-locking: It still CASes the lowest two > header bits to 00 to indicate 'fast-locked' but does *not* overload the upper > bits with a stack-pointer. Instead, it pushes the object-reference to a > thread-local lock-stack. This is a new structure which is basically a small > array of oops that is associated with each thread. Experience shows that this > array typcially remains very small (3-5 elements). Using this lock stack, it > is possible to query which threads own which locks. Most importantly, the > most common question 'does the current thread own me?' is very quickly > answered by doing a quick scan of the array. More complex queries like 'which > thread owns X?' are not performed in very performance-critical paths (usually > in code like JVMTI or deadlock detection) where it is ok to do more complex > operations (and we already do). The lock-stack is also a new set of GC roots, > and would be scanned during thread scanning, possibly concurrently, via the > normal p rotocols. > > The lock-stack is fixed size, currently with 8 elements. According to my > experiments with various workloads, this covers the vast majority of > workloads (in-fact, most workloads seem to never exceed 5 active locks per > thread at a time). We check for overflow in the fast-paths and when the > lock-stack is full, we take the slow-path, which would inflate the lock to a > monitor. That case should be very rare. > > In contrast to stack-locking, fast-locking does *not* support recursive > locking (yet). When that happens, the fast-lock gets inflated to a full > monitor. It is not clear if it is worth to add support for recursive > fast-locking. > > One trouble is that when a contending thread arrives at a fast-locked object, > it must inflate the fast-lock to a full monitor. Normally, we need to know > the current owning thread, and record that in the monitor, so that the > contending thread can wait for the current owner to properly exit the > monitor. However, fast-locking doesn't have this information. What we do > instead is to record a special marker ANONYMOUS_OWNER. When the thread that > currently holds the lock arrives at monitorexit, and observes > ANONYMOUS_OWNER, it knows it must be itself, fixes the owner to be itself, > and then properly exits the monitor, and thus handing over to the contending > thread. > > As an alternative, I considered to remove stack-locking altogether, and only > use heavy monitors. In most workloads this did not show measurable > regressions. However, in a few workloads, I have observed severe regressions. > All of them have been using old synchronized Java collections (Vector, > Stack), StringBuffer or similar code. The combination of two conditions leads > to regressions without stack- or fast-locking: 1. The workload synchronizes > on uncontended locks (e.g. single-threaded use of Vector or StringBuffer) and > 2. The workload churns such locks. IOW, uncontended use of Vector, > StringBuffer, etc as such is ok, but creating lots of such single-use, > single-threaded-locked objects leads to massive ObjectMonitor churn, which > can lead to a significant performance impact. But alas, such code exists, and > we probably don't want to punish it if we can avoid it. > > This change enables to simplify (and speed-up!) a lot of code: > > - The inflation protocol is no longer necessary: we can directly CAS the > (tagged) ObjectMonitor pointer to the object header. > - Accessing the hashcode could now be done in the fastpath always, if the > hashcode has been installed. Fast-locked headers can be used directly, for > monitor-locked objects we can easily reach-through to the displaced header. > This is safe because Java threads participate in monitor deflation protocol. > This would be implemented in a separate PR > > Also, and I might be mistaken here, this new lightweight locking would make > synchronized work better with Loom: Because the lock-records are no longer > scattered across the stack, but instead are densely packed into the > lock-stack, it should be easy for a vthread to save its lock-stack upon > unmounting and restore it when re-mounting. However, I am not sure about > this, and this PR does not attempt to implement that support. > > Testing: > - [x] tier1 x86_64 x aarch64 x +UseFastLocking > - [x] tier2 x86_64 x aarch64 x +UseFastLocking > - [x] tier3 x86_64 x aarch64 x +UseFastLocking > - [x] tier4 x86_64 x aarch64 x +UseFastLocking > - [x] tier1 x86_64 x aarch64 x -UseFastLocking > - [x] tier2 x86_64 x aarch64 x -UseFastLocking > - [x] tier3 x86_64 x aarch64 x -UseFastLocking > - [x] tier4 x86_64 x aarch64 x -UseFastLocking > - [x] Several real-world applications have been tested with this change in > tandem with Lilliput without any problems, yet > > ### Performance > > #### Simple Microbenchmark > > The microbenchmark exercises only the locking primitives for monitorenter and > monitorexit, without contention. The benchmark can be found > (here)[https://github.com/rkennke/fastlockbench]. Numbers are in ns/ops. > > | | x86_64 | aarch64 | > | -- | -- | -- | > | -UseFastLocking | 20.651 | 20.764 | > | +UseFastLocking | 18.896 | 18.908 | > > > #### Renaissance > > | x86_64 | | | | aarch64 | | > -- | -- | -- | -- | -- | -- | -- | -- > | stack-locking | fast-locking | | | stack-locking | fast-locking | > AkkaUct | 841.884 | 836.948 | 0.59% | | 1475.774 | 1465.647 | 0.69% > Reactors | 11041.427 | 11181.451 | -1.25% | | 11381.751 | 11521.318 | -1.21% > Als | 1367.183 | 1359.358 | 0.58% | | 1678.103 | 1688.067 | -0.59% > ChiSquare | 577.021 | 577.398 | -0.07% | | 986.619 | 988.063 | -0.15% > GaussMix | 817.459 | 819.073 | -0.20% | | 1154.293 | 1155.522 | -0.11% > LogRegression | 598.343 | 603.371 | -0.83% | | 638.052 | 644.306 | -0.97% > MovieLens | 8248.116 | 8314.576 | -0.80% | | 7569.219 | 7646.828 | -1.01%% > NaiveBayes | 587.607 | 581.608 | 1.03% | | 541.583 | 550.059 | -1.54% > PageRank | 3260.553 | 3263.472 | -0.09% | | 4376.405 | 4381.101 | -0.11% > FjKmeans | 979.978 | 976.122 | 0.40% | | 774.312 | 771.235 | 0.40% > FutureGenetic | 2187.369 | 2183.271 | 0.19% | | 2685.722 | 2689.056 | -0.12% > ParMnemonics | 2434.551 | 2468.763 | -1.39% | | 4278.225 | 4263.863 | 0.34% > Scrabble | 111.882 | 111.768 | 0.10% | | 151.796 | 153.959 | -1.40% > RxScrabble | 210.252 | 211.38 | -0.53% | | 310.116 | 315.594 | -1.74% > Dotty | 750.415 | 752.658 | -0.30% | | 1033.636 | 1036.168 | -0.24% > ScalaDoku | 3072.05 | 3051.2 | 0.68% | | 3711.506 | 3690.04 | 0.58% > ScalaKmeans | 211.427 | 209.957 | 0.70% | | 264.38 | 265.788 | -0.53% > ScalaStmBench7 | 1017.795 | 1018.869 | -0.11% | | 1088.182 | 1092.266 | > -0.37% > Philosophers | 6450.124 | 6565.705 | -1.76% | | 12017.964 | 11902.559 | > 0.97% > FinagleChirper | 3953.623 | 3972.647 | -0.48% | | 4750.751 | 4769.274 | > -0.39% > FinagleHttp | 3970.526 | 4005.341 | -0.87% | | 5294.125 | 5296.224 | -0.04%
Roman Kennke has updated the pull request incrementally with one additional commit since the last revision: Disable new lightweight locking in Zero ------------- Changes: - all: https://git.openjdk.org/jdk/pull/10907/files - new: https://git.openjdk.org/jdk/pull/10907/files/43cdbb53..82b8b702 Webrevs: - full: https://webrevs.openjdk.org/?repo=jdk&pr=10907&range=73 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10907&range=72-73 Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod Patch: https://git.openjdk.org/jdk/pull/10907.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/10907/head:pull/10907 PR: https://git.openjdk.org/jdk/pull/10907
